The present invention relates generally to retroreflective articles such as sheeting. More particularly, the invention relates to such articles or sheetings in which retroreflective elements comprise reflective faces arranged to form a cavity.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
Cube corner retroreflective sheetings can generally be categorized as those that use a rear-surface body layer and those that use a front-surface body layer. Commercially available cube corner retroreflective sheetings are of the former type, in which a thin transparent body layer has a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures of pyramidal shape, some or all of which include three reflective faces configured as a cube corner element. Light is incident on the planar front surface, passes through the thickness of the body layer, and is retroreflected by the cube corner elements back through the front surface. In some known embodiments, a reflective coating such as aluminum is applied to the rear structured surface, followed by an adhesive layer that covers and conforms to some extent to the shape of the structured surface. However, in general no reflective coating is required so long as a clean air interface can be maintained at the structured surface, in which case reflections occur by total internal reflection.
Some known cube corner retroreflective sheeting constructions use a front-surface body layer, in which the body layer has a front structured surface. See, e.g., U.S. Pat. No. 3,712,706 (Stamm), U.S. Pat. No. 4,127,693 (Lemelson), and U.S. Pat. No. 4,656,072 (Coburn, Jr. et al.), and PCT Publication WO 89/06811 (Johnson et al.). The front structured surface comprises a plurality of reflective faces arranged to form cube corner cavities. For this reason such retroreflective sheeting is referred to herein as cube corner cavity based retroreflective sheeting. A thin metal film can be applied to the structured surface to enhance reflectivity of the faces. Incident light does not penetrate through the body layer but rather is reflected by the faces forming the cube corner cavities. In some embodiments a cover layer that does transmit incident light is provided on top of the structured surface to protect the cavities from dirt or other degradation, with portions of the cover layer extending into and filling in the cube corner cavities of the structured surface. In other embodiments a cover layer is sealed or adhered to the structured surface by a colored pressure- or heat-sensitive adhesive that cancels, removes, or obliterates retroreflectivity of the structured surface.
One advantage of cube corner cavity-based retroreflective sheeting is the ability to use a much wider variety of material compositions for the body layer than is otherwise possible, since it need not be optically clear. Another advantage is the ability to form certain types of structured surfaces in the body layer more rapidly than it takes to form a negative copy of such structured surfaces in rear-surface body layer constructions. This is because molds used to form the structured surface of a front-surface body layer can have grooves that are essentially unbounded in the direction of the groove. In contrast, molds used to form the structured surface of a rear-surface body layer typically have an array of closed (cube corner) cavities bounded by a plurality of inverted grooves, i.e., ridges. The unbounded grooves of the former molds are easier to fill with body layer material than the array of closed cavities provided on the latter molds.
Unfortunately, this latter advantage can be essentially nullified in constructions where the cube corner cavities in the body layer are filled with a transparent substance. Filling the cavities with such a substance, referred to as a fill material, is desirable to increase the entrance angularity of the sheeting by refracting highly off-axis incident light closer to the symmetry axis of the cube corner element, as well as to keep dirt or other debris out of the cavities. But such filling is undesirable insofar as it requires forcing material into an array of closed cavities. Such filling is also undesirable to the extent it exposes the body layer to excessive heat, mechanical stress, or other process conditions that could compromise the fidelity of the structured surface.
Constructions of the type described would benefit from fill materials having properties that make them easy to fill into the cube corner cavities of the body layer, preferably with minimal risk of damaging the fidelity of the structured surface. Preferred fill materials should be compatible with relatively low cost, high flexibility, and high visibility sheeting constructions.